Abstract

The congestion probe feature of the Orange County Transportation Authority (California) bus probe project was evaluated by comparing automobile and bus trajectories and examining alternative congestion detection methods. The focus was city streets on which delays occur at signalized intersections and bus delays at bus stops. The analysis revealed that when automobiles have long delays, buses traveling nearby on the same route are also likely to be delayed. The reverse situation, however, is not always true, because buses frequently wait for extended periods when they run ahead of schedule. Any useful bus probe algorithm needs to distinguish between actual congestion and a stopping delay. Although the transit probe was designed to measure congestion on roadway segments, a more useful approach would be to measure congestion approaching major intersections, where delays are likely to occur. Moreover, because delays randomly fluctuate according to a vehicle's arrival time relative to the signal cycle, the most sensible approach is to set off a "congestion alarm" when a vehicle is delayed by more than one cycle at an intersection. A congestion alarm would indicate oversaturation and delay well above normal.